The present invention relates to a high frequency circuit with variable phase shift, usable mainly in the field of decimetric waves. It can be used therein in particular together with a plurality of antenna elements to provide variable squint pointing by feeding each of the antennas with the same signal for transmission, and by controlling the phase shifter circuits associated with the antennas to take up a determined phase relating to the pointing to be achieved. Nevertheless, other applications are possible. The object of the invention is to use a signal of given phase at the output from a circuit to produce a signal of phase that is offset relative to said given phase. The principle of the invention is also applicable to the field of non-decimetric waves.
In the field of phase shifters, phase shifting circuits are known, in particular those based on so-called PIN diodes. PIN diodes are constituted by juxtaposing a P layer and an N layer of semiconductor material on either side of a thin insulating layer. Because of the presence of the insulating layer, the minority carriers of the PN junction are slow. Compared with a very high frequency signal, such a diode, when properly biased, can thus behave like a circuit exhibiting pure resistance.
Some publications describe PIN diodes as constituting switch elements at microwave frequencies. Such an element makes it possible to provide on/off selection of one particular transmission access selected from two possible accesses. In other applications, such diodes make it possible to connect a segment of line for reflection purposes in parallel with a given line. Such applications in the form of two-state functions are tied to the fact that with such diodes, insertion losses and standing wave ratios (SWRs) at the accesses can be controlled and defined simultaneously only in the two switching states. Specifically, that type of circuit can be guaranteed to be reproducible and suitable for industrialization only providing it is not used at settings that are intermediate between those two states. Known methods do not make it possible to provide continuous and simultaneous control over phase, standing wave ratio, and insertion loss.
Various types of circuit based on multiple Varactor diodes or indeed based on multiple PIN diodes have been used to make variable-shift phase shifters. The problem presented by Varactor diodes is that the capacitance of such diodes varies with bias voltage. They also have the drawback, particularly in the 3 GHz range, that the voltages required for scanning significant variation of impedance need excursions of the order of 20 volts. Such excursions are quite difficult to implement, even with voltage multipliers using charge pumps. In addition, Varactor diodes give rise to variations in reactive impedance that are difficult to compensate, unless some other reactive impedance is also used.
As for PIN diodes, which have the advantage of proposing variation that is of a resistive type, they nevertheless need to be manufactured with great care in order to be usable beyond 3 GHz, because of the presence of parasitic capacitance. This parasitic capacitance gives rise to a limit on frequency since the diode is connected in series to convey the radio signal. In addition, the use of circuits having numerous PIN diodes implies making microwave frequency circuits that occupy a large area on a printed circuit board, which is bulky and more difficult to develop. In particular, specialized PIN diodes are diodes in ceramic packages that are mounted manually. Under such circumstances, these ceramic packages are not surface mount components (SMC) type packages that are suitable for being put into place automatically by insertion machines in mass-produced circuits. Furthermore, the connection tabs of such packages gives rise to inductances which, in combination with the parasitic capacitance of the diode, can make such a circuit very difficult to define.
With multiple Varactor or PIN diodes, expensive and complex solutions requiring 90xc2x0 couplers or 3 dB couplers must therefore be used in order to maintain good matching of input and output impedances.
An object of the invention is to remedy those problems of expense and of adjustment in particular, by proposing a solution in which the diodes used are PIN diodes (having slow minority carriers) of conventional type, i.e. PIN diodes of the kind that are available in packages suitable for surface mounting using automatic machines.
One idea of the invention is to provide, between an inlet and an outlet of a phase shifter, a separation into two propagation paths of different lengths. In addition, at least one PIN diode, and in practice two PIN diodes in parallel, are interconnected at intermediate positions via their terminals to nodes of each of these paths. By biasing the diode and ensuring that it presents a given resistance, a bias circuit makes it possible for each of the two paths to transfer an impedance to the input that will be seen by the signal at the input. Consequently, the input signal will take one path rather than the other. Since the paths are of different lengths, the two resulting signals at the output are phase-shifted relative to each other. When they are combined, they give rise to a signal which is the result of adding them together, and which possess a phase that depends on the respective contributions of each of these two components. The more favored signal imposes its phase the more easily.
In practice, such a phase shifter can produce a phase shift of about 20xc2x0. That is entirely satisfactory for pointing the aiming direction of an antenna having a plurality of radiating elements onto an off-axis or xe2x80x9csquintxe2x80x9d direction. If a phase shift of greater than 20xc2x0 is desired, then it suffices to cascade a plurality of phase shifters of the same type as the phase shifter of the invention.
It is shown below that compared with the state of the art, the circuit of the invention presents the advantage that the signal to be transmitted does not pass via the PIN diodes. As a result, the parasitic capacitances of the diodes does not complicate the operation of the circuit. In practice, the imaginary impedance components of the PIN diodes are compensated by matching circuits, by metallic connections of desired length. Such matching has the advantage of being effective over a very wide range of use. For example, in a given circuit operating at around 6.6 GHz, it is very easy to use the phase shifter between 6.2 GHz and 6.9 GHz, i.e. over a range of more than 10% of the center of frequency.
The invention thus provides a variable phase shift high frequency circuit comprising an input for a high frequency signal, two propagation paths for said signal each connected at one end to said input, a PIN diode having its terminals connected to first and second intermediate nodes on each of the two paths respectively, an output for the phase-shifted high frequency signal connected to the other ends of the two paths, and a circuit for biasing the diode.